Re: MTRR setup in OVMF [was: PATCH v3 01/10 KVM: MMU: fix decoding cache type from MTRR]

Jordan Justen
2015-07-14T22:37:09Z

On 2015-07-14 14:29:11, Laszlo Ersek wrote:
> On 07/14/15 23:15, Paolo Bonzini wrote:
> >> The long delay that Alex reported (for the case when all guest memory
> >> was set to UC up-front) is due to the fact that the SEC phase of OVMF
> >> decompresses an approximately 1712 KB sized, LZMA-compressed blob, to
> >> approx. 896 KB worth of PEI drivers and 8192 KB worth of DXE and UEFI
> >> drivers -- and this decompression is extremely memory-intensive.
> >>
> >> (When Jordan implemented that reset vector first, we saw similar
> >> performance degradation on AMD hosts (albeit not due to MTRR but due to
> >> page attributes). See
> >> <https://github.com/tianocore/edk2/commit/98f378a7>. I'm only mentioning
> >> it here because it makes me appreciate the current problem report.)
> >>
> >> Anyway, the reset vector's page table building is implemented in
> >> "OvmfPkg/ResetVector/Ia32/PageTables64.asm". The decompression in SEC
> >> can be found in "OvmfPkg/Sec/SecMain.c", function DecompressMemFvs().
> >
> > Perhaps the OVMF reset vector should initialize the MTRRs for the BSP?
>
> That's an idea, yes, if someone feels sufficiently drawn to writing
> assembly.

Maybe we can use MtrrLib in the SEC C code?

-Jordan

> Complications:
> - the reset vector is specific to OvmfPkg only in the OvmfPkgX64.dsc
> build
> - it needs to be determined what memory to cover.
>
> > I think SEC doesn't do any MMIO, so it should be enough to enable MTRRs
> > and set the default type to writeback.
>
> Seems safe to me, off the top of my head (and testing could confirm /
> disprove quickly).
>
> > In any case we're going to have to quirk it, because of the broken
> > guests in the wild.
>
> Thanks.
> Laszlo